1. Field
This document relates to a display device, and more particularly, to an organic light emitting diode display device and a driving method thereof.
2. Related Art
Recently, various flat display panel technologies have become more common due to reduced weight and bulk in comparison to cathode ray tube (CRT) technology. Such flat display panel technologies include liquid crystal displays, field emission displays, plasma display panels, and electro-luminescence (EL) display devices.
Among these, the EL display device is a self-luminous device that causes a fluorescent substance to emit light by a re-combination of an electron and a hole, and can be generally classified into an inorganic EL where an inorganic compound is used as the fluorescent substance and an organic EL where an organic compound is used. The EL display device has many advantages such as low driving voltage self-luminescence, thin profile, wide-viewing angle, rapid response speed, and high contrast. Hence, the EL device is expected to be a next generation display device.
The organic EL device generally includes an electron injection layer, an electron transport layer, a light-emitting layer, a hole transport layer, and a hole injection layer. In such an organic EL device, when a specified voltage is applied between an anode and a cathode, an electron generated from the cathode moves to the light-emitting layer through the electron injection layer and the electron transport layer. Meanwhile, a hole generated from the anode moves to the light-emitting layer through the hole injection layer and the hole transport layer. Accordingly, the re-combination of the electron and the hole supplied from the electron transport layer and the hole transport layer causes light to be emitted in the light-emitting layer.
The circuit configuration of each pixel of a general organic light emitting diode display device using such an organic EL will be discussed with reference to FIG. 1.
FIG. 1 is an equivalent circuit diagram of a pixel of a general organic light emitting diode display device.
Referring to FIG. 1, each pixel of the organic light emitting diode display device comprises a switching thin film transistor S_TR1 turned on by a scan pulse supplied through a gate line GL and for switching a data voltage supplied through a data line DL, a storage capacitor Cst for charging the data voltage supplied through the switching thin film transistor S_TR1, an organic light emitting diode OLED turned on by a driving current supplied from a power supply terminal to which a high potential power voltage VDD, and a driving thin film transistor D_TR1 turned on by the data voltage supplied through the switching thin film transistor S_TR1 or the charge voltage of the storage capacitor Cst and for driving the organic light emitting diode OLED.
The switching thin film transistor S_TR1 is an N-MOS thin film transistor having a gate connected to the gate line GL, a drain connected to the data line DL, and a source commonly connected to the storage capacitor Cst and the gate of the driving thin film transistor D_TR1. The switching thin film transistor S_TR1 is turned on by the scan pulse supplied through the gate line GL to supply the data voltage supplied through the data line DL to the storage capacitor Cst and the driving thin film transistor D_TR1.
The storage capacitor Cst has one side commonly connected to the switching thin film transistor S_TR1 and the gate of the driving thin film transistor D_TR1 and other side connected to a ground, and is charged with the data voltage supplied through the switching thin film transistor S_TR1. The storage capacitor Cst discharges the charged voltage when the data voltage being supplied through the switching thin film transistor S_TR1 is stopped to be applied to the gate of the driving thin film transistor D_TR1, that is, when the gate voltage of the driving thin film transistor D_TR1 starts to be dropped, thereby holding the gate voltage of the driving thin film transistor D_TR1. Accordingly, even if the supply of the data voltage supplied through the switching thin film transistor S_TR1 is stopped, the driving thin film transistor D_TR1 keeps a turned-on state by the charge voltage of the storage capacitor Cst during the holding period by the storage capacitor Cst.
The organic light emitting diode OLED has an anode connected to the power supply terminal applied with the high potential power voltage VDD and a cathode connected to the drain of the driving thin film transistor D_TR1.
The driving thin film transistor D_TR1 is an N-MOS thin film transistor having a gate commonly connected to the source of the switching thin film transistor S_TR1 and the switching transistor S_TR1, a drain connected to the cathode of the organic light emitting diode OLED, and a source connected to the ground. The driving thin film transistor D_TR1 is turned on by the data voltage supplied to the gate via the switching thin film transistor S_TR1 and the charge voltage of the switching thin film transistor S_TR1 supplied to the gate and switches a driving current flowing in the organic light emitting diode OLED over to the ground, thereby allowing the organic light emitting diode OLED to emit light by the driving current generated by the high potential power voltage VDD.
Since the conventional organic light emitting diode display device with pixels having an equivalent circuit employs one driving thin film transistor, there is a problem that the driving thin film transistor is deteriorated due to a stress by a bias continuously applied to the gate of the driving thin film transistor.
In order to solve this problem, there has been developed a conventional organic light emitting diode display device which has two driving thin film transistors formed in each pixel, and the two driving thin film transistors provided in each pixel are alternately driven so as to reduce the stress caused by the bias. The conventional organic light emitting diode display device of this type supplies a high potential power voltage VDD, the driving voltage of an organic light emitting diode, to the organic light emitting diode of each pixel through one power supply line formed on a display panel (not shown), and thus the high potential power voltage VDD is dropped due to the resistance component of the power supply line and supplied to each pixel. By the dropping of the high potential power voltage VDD, the conventional organic light emitting diode display device with two thin film transistors formed in each pixel is unable to represent a desired gray level for each pixel.